Recs.
Updated
A toolkit providing simple and flexible HTML, CSS, and JS for popular UI components and interactions.
Specs
Pros
Pro Well documented
Documentation is thorough, well organized and full of live examples and templates ready for use. Every component and every part of the framework is explained and covered in depth.
Pro A wide variety of themes available
To help you avoid the "Bootstrap look", there are many resources that provide a great selection of themes and templates for Bootstrap.
Pro Custom builds
If you don't want to download the full Bootstrap framework. Custom builds of Bootstrap can be created, including only the desired CSS, CSS components, and JavaScript components. This can be done directly from the Bootstrap website by simply choosing what components to download.
Cons
Con Too many classes
Bootstrap's over-reliance on HTML classes for styling can get very messy very quickly. There's also an overabundance of DOM elements which have a lot of classes and are more often than not nested inside DOM elements with even more classes. This gets problematic down the line because the maintainability of the project gets harder when the project starts to get large.